In stark contrast to the transition metals, examples of imido or alkylidene complexes of the lanthanides remain scarce. A recent literature survey reveals that only nine examples of lanthanide imido complexes have been reported, and the majority of these have arisen serendipitously. Concrete examples of species containing lanthanide-carbon multiple bonds are even more sparse. Recently, some rational approaches to the synthesis of lanthanide complexes containing Ln=X functionalities have been detailed (X = C, N). Additionally, a DFT (Density Functional Theory) study of a samarium imido complex has provided insight into the electronic and steric factors that may be necessary to support these unusual reactive groups. This Perspective reviews the work in this field and offers some suggestions to expand this potentially useful class of compounds.

The preparation of new scandium phosphine complexes that contain two hydrocarbyl groups is reported. Thus, reaction of the amido diphosphine ligand precursor LiN(SiMe2CH2PPri 2)2 with ScCl3(THF)3 in toluene at 100 °C leads to the formation of ScCl2(THF)[N(SiMe2CH2PPri 2)2] for which the THF molecule can be removed by pumping in the solid state. The X-ray crystal structure of this molecule shows it to be monomeric with a distorted octahedral geometry having trans-disposed chloride ligands and the tridentate ligand meridionally bound. The solution NMR spectra are also consistent with this geometry. Addition of the alkyllithium reagents RLi (where R = Me, Et, and CH2SiMe3) leads to the formation of bis(hydrocarbyl) derivatives of the formula ScR2[N(SiMe2CH2PPri 2)2]. These are the first dialkyl-substituted scandium complexes that have been characterized. The solid-state X-ray structures of ScEt2[N(SiMe2CH2PPri 2)2] and Sc(CH2SiMe3)2[N(SiMe2CH2PPri 2)2] show that these molecules are mononuclear in the solid state with distorted trigonal bipyramidal geometries. In solution, the NMR spectroscopic parameters are consistent with overall C 2 v symmetry. A number of reactions were attempted such as addition of CO, H2, CH3I, nitriles, isocyanides, and silanes:  in all cases, complete decomposition to a mixture of unidentifiable products was observed. The reaction of ethylene with these bis(hydrocarbyl) species did result in the formation of polyethylene, but the nature of the catalytically active species could not be determined. For the reaction with CO2, insertion was observed to be competitive with decomposition. Molecular orbital calculations show that the frontier orbitals of the five-coordinate, symmetrized complex Sc(CH3)2[N(SiMe2CH2PH2)2] (isopropyl groups at phosphorus replaced with H's) have a different symmetry than that of metallocene systems; in addition, the HOMO was found to be largely the amido nitrogen lone pair of the Sc−N interaction.

La[N(SiMe 3 ) 2 ] 3 reacted with 1,3-dicyclohexylcarbodiimide in refluxing toluene to yield the mono-guanidinate complex La[CyNC(N(SiMe 3 ) 2 )NCy](N(SiMe 3 ) 2 ) 2 1. Compound 1 is monomeric in solution; X-ray structural analysis reveals an unassociated complex in the solid state, with a four-coordinate lanthanum center. Complex 1 reacts with 2,6-di-tert-butylphenol (2 equivalents) in cold pentane to yield the bis(phenoxide) complex La[CyNC(N(SiMe 3 ) 2 )NCy]-(OC 6 H 3 t Bu 2 -2,6) 2 . X-Ray analysis indicates a similar structure to that of 1, with a four-coordinate lanthanum center coordinated by a single guanidinate ligand and two phenoxide groups. This compound catalyses the ring-opening polymerization of ,-lactide to polylactide. Although high molecular weight polymer is obtained, polydispersities are broad and no control over the stereochemistry of the polymer is observed.

Reaction of 3 equiv of 2,6-diisopropylaniline with Sm[N(SiMe 3 ) 2 ] 3 affords the dimeric species [Sm(NHAr) 3 ] 2 (1). X-ray crystallography illustrates that each metal center in 1 engages in an η 6 -arene interaction with the aryl ring of an amide ligand attached to an adjacent samarium. IR spectroscopy indicates that the π-arene interactions are maintained in solution. Reaction of 1 with 4 equiv of trimethylaluminum leads to formation of the bis(µ 2 -imido) complex [(µ-ArN)Sm(µ-NHAr)(µ-Me)AlMe 2 ] 2 (2). The molecular structure of 2 contains a unique central Sm 2 N 2 core which displays extremely short bridging Sm-N distances of 2.152-(8) and 2.271(7) Å, characteristic of an imido complex. Density functional theory (DFT) calculations have been carried out in order to gain a better understanding of the nature of the bonding interactions within complex 2 and indicate that the 5d metal acceptor orbitals play a significant role in stabilizing π-donation from the imido groups to the samarium centers within the Sm 2 N 2 core.